added material budget to the documentation

(cherry picked from commit 2481f48d)

doc/usermanual/chapters/configuration.tex

@@ -290,7 +290,8 @@ All supported rotations are extrinsic active rotations, i.e. the vector itself i
 \item The \parameter{time_offset} can be used to shift the individual detector time frames of reference to e.g.\ account for time of flight effects between different detector planes by adding a fixed offset.
 \item The \parameter{material_budget} defines the material budget of the
   sensor layer in factions of the radiation length, including support
-  structures. 
+  structures. If no value is defined a default of zero is assumed. A given
+  value has to be larger than zero. 
 \item Pixels to be masked in the offline analysis can be placed in a separate file specified by the \parameter{mask_file} parameter explained in detail in Section~\ref{sec:masking}.
 \item A region of interest in the given detector can be defined using the \parameter{roi} parameter. More details on this functionality can be found in Section~\ref{sec:roi}.
 \end{itemize}